a) Field of the Invention
The present invention relates to a zoom type imaging optical system which is to be used in a condition where it is connected to an eyepiece of an endoscope.
b) Description of the Prior Art
It is generally practiced to connect TV cameras or film cameras to eyepieces of endoscopes for performing diagnosis by observing images which are formed by the endoscopes and projected onto the TV cameras or recorded on the films. In the recent days, it is widely practiced to perform medical diagnosis and treatments while observing images which are formed by endoscopes and projected onto TV monitors by using small TV cameras, especially those incorporating the solid-state image pickup devices such as CCD.
Since the solid-state image pickup devices are designed very small and highly integrated by the recent progress made in the field of the semiconductor technology, the imaging optical systems to be used in combination with the solid-state image pickup devices must have high performance.
FIG. 1 shows a configuration of a system for observing an image formed by an endoscope on a TV monitor. In this drawing, an eyepiece 2 of an endoscope 1 incorporating an objective lens system 1, an image guide (image transmitting optical system), etc. is connected to an adapter 3 having an imaging optical system 4 and a TV camera 5 having a solid-state image pickup device 6, which in turn is connected to a camera control unit 7 and a monitor 8.
In the system having the configuration described above, an image of object formed by the eyepiece 2 arranged in the endoscope 1 is focused onto the solid-state image pickup device 6 by the imaging optical system 4 built in the adapter 3 and electric signals provided from the solid-state image pickup device are processed by the camera control unit 7 so that the image is projected onto the monitor 8 for observation.
For observation of images formed by the system shown in FIG. 1, several kinds of adapters having different magnifications are prepared so that they are to be used selectively dependently on types of endoscopes to be employed and purposes of observations. Therefore, the system requires a multiple number of adapters and a high manufacturing cost. In addition, when it is desired to observe an image at different magnifications, the system requires exchanging the adapters each time the magnification is to be changed from one to another though it is rather difficult, for example during medical operation, to exchange the adapters from one to another. Even when the adapters can be exchanged during the medical operation, it will be difficult to project an image at the size desired by the surgical operator.
In order to correct the defect described above, imaging optical systems of adapters are designed as zoom optical systems so as to permit freely change sizes of images of locations to be observed on the TV monitor.
On the other hand, most eyepiece lens systems for a definite type of endoscopes are designed so as to have a certain definite imaging point, or a fixed dioptric power when connected to adapters.
When a zoom imaging optical system is combined with eyepiece lens systems for endoscopes having a certain definite diopter, object points for the adapter are located at the same position since the eyepiece lens systems have the same dioptric power. Accordingly, the zoom imaging optical system requires no focusing as exemplified by the zoom imaging optical system disclosed by Japanese Patent Kokai Publication No. Hei 1-128031.
However, eyepiece lens systems for endoscopes such as non-flexible endoscopes have dioptric powers which are different dependently on types of the endoscopes.
When the zoom imaging optical system is to be combined with eyepiece lens systems for endoscopes having different dioptric powers, it is necessary to equip the zoom imaging optical system with a focusing mechanism.
As a method for focusing the zoom imaging optical system, there is known the method adopted by the optical system disclosed by U.S. Pat. No. 4,781,448 which moves the optical system as a whole along the optical axis for focusing. This focusing method allows the moving distance to be changed along with variation of focal length of the optical system caused by focusing. When an eyepiece lens system having a different dioptric power is combined with the zoom imaging optical system, moving distance of a lens unit required for adjusting dioptric power is variable dependently on magnification of the latter. When the focal length at the tele position of the imaging optical system is twice as long as that at the wide position thereof, for example, the moving distance of the lens unit required for adjusting the same number of diopters is prolonged four times as long. Therefore, it is necessary to reserve wide airspaces before and after the optical system, thereby enlarging the adapter as a whole.
Further, since numbers of diopters to be adjusted varies along with variation of magnification of the zoom imaging optical system, it is impossible to adopt a scale or a similar member for indicating numbers of diopters to be adjusted. Therefore, it is impossible to know the moving distance of the movable lens unit required for focusing an endoscope even when the imaging optical system is combined with an eyepiece lens system for endoscopes having a known dioptric power, thereby obliging it to perform focusing the endoscope while observing an actual image on the TV monitor.
Furthermore, since the adapter incorporating the zoom imaging optical system must be as small as possible in its external design and as light as possible in its weight since the adapter is connected to endoscopes and operated integrally therewith. Such compact design is important not only from the viewpoint of operation but also for lessening the fatigue of the operator.
There is currently available an endoscope which is composed, as shown in FIG. 2, of an observation system consisting of an objective lens system 11 and an image transmitting system such as an image guide 12, and an illumination system comprising a light guide 13, but equipped with no eyepiece lens system.
The endoscope of this type which is equipped with no eyepiece lens system can be manufactured at a low cost, and is preferrable especially for the medical field since it can be discarded, once it is used, to assure excellent hygenical effect. In order to project an image formed by the endoscope of this type onto a TV monitor, it is necessary to equip the endoscope with a lens which has the function equivalent to that of an eyepiece lens system.